In a typical MRI apparatus, an RF coil is typically used to sense the imaging information. The RF coil comprises a number of imaging elements and receive coils corresponding to the imaging elements. The RF coil is connected to one or more image reconstruction engines through a number of co-axial cables depending on the number of receive coils and imaging elements in the RF coil. The proximity of the receive coils and cables with one another can lead to ghosting, low signal to noise ratio and even standing waves at times. These become more significant in multi-RF channel parallel imaging applications.
A viable solution to the issues listed above would be to eliminate the co-axial cables running between the receive coils and the image reconstruction engine thereby employing wireless transmission of MR data between the RF (receive) coil and the image reconstruction engine. There have been experiments on transmission of MR data by wireless means using amplitude modulation, direct frequency modulation and digital modulation techniques.
The integrity of the MR signal's amplitude and phase sensed by the receive coil needs to be preserved for accurate reconstruction of the image. One limitation associated with amplitude modulation is, an amplitude-modulated signal is most susceptible to noise, amplitude and phase non-linearities of active semiconductor devices involved in the amplifier and mixer stages.
On the other hand, frequency modulation is less susceptible to RF channel noise, amplitude and phase non-linearities as the demodulator's fidelity depends largely on frequency variations. However, the frequency modulation technique consumes a huge bandwidth. Another limitation associated with frequency modulation is, implementation of direct frequency modulation may require high stable and monotonic voltage controlled oscillators (VCOs). In addition it may be necessary to include frequency stabilization circuits, such as a closed loop frequency control system, to ensure that the modulation of the VCO does not render its frequency unstable. Further, the modulation index could be different for different centre frequencies of the VCO.
Yet another option of using digital modulation would require a high dynamic range Analog To Digital Converter (ADC) on each RF channel with associated sampling circuits, sampling clock circuits and other relevant digital circuits on the RF coil. One limitation associated with this technique is, the process of sampling and conversion of the MR signal to equivalent digital information may be time consuming.
Another limitation associated with the digital modulation technique is, because the analog-to-digital converter has a finite dynamic range, substantial amplitude distortion in the receive chain can result in overloading the analog-to-digital converter producing a clipped digital signal or otherwise erroneous digitized data. Similarly, when the signal is at lower end of the dynamic digitizing range, the digitizing process can introduce substantial digitization noise.
It is therefore necessary to build a communication architecture that can have high fidelity of reproduction of modulation signal with high frequency stability, insusceptible to external electromagnetic interferences, feasible and economical to build in large numbers and easy to maintain.